This invention relates to polymerizable compositions for in-mold curing and in-mold cured products obtained from such compositions.
Polymerizable compositions are widely in use. For example, molded products from polymerizable compositions containing unsaturated polyester resin have been used as external panels and accessaries of automobiles. There are a number of molding methods available for obtaining molded objects from polymerizable composition, including the compression molding method using sheet molding compound (SMC), the injection molding method using bulk molding compound (BMC), the hand lay-up method whereby polymerizable compositions impregnate a reinforcing fiber mat, textile or strands and is hardened, and the pultrusion method. In addition, examples of method known as the most ideal and economical for obtaining molded objects include the resin transfer molding (RTM) and reaction injection molding (RIM) whereby a polymerizable composition is introduced into a mold for quick in-mold curing.
As examples of prior art polymerizable compositions containing unsaturated urethane, U.S. Pat. No. 4,480,079 and European Patent Publication EP197682 have disclosed polymerizable composition comprising unsaturated urethane and alkyl (meth)acrylate obtained from polyisocyanate and hydroxy alkyl (meth)acrylate.
The aforementioned prior art polymerizable compositions, however, have the following disadvantages because of their use of unsaturated urethane obtained from polyisocyanate and hydroxy alkyl (meth)acrylate:
(1) Since unsaturated urethane and alkyl (meth)acrylate, which are obtained together by such a prior art method, are not sufficiently compatible with respect to each other, sedimentation and separation are likely to take place especially at low temperatures;
(2) Since unsaturated urethane thus obtained contains many urethane bonds within its molecule, it is inferior in water resistance, like most other kinds of polyurethane liquid resin, as compared to other thermosetting polymerizable compositions. This limits the use of the in-mold cured products obtained therefrom; and
(3) Since the density of cross-links is high in such in-mold cured products, mold shrinkage at the time of their curing is large compared to unsaturated polyester liquid resins. As a result, generation of cracks and appearance of fiber patterns are prominently observed. Low shrinkage agents may be used for preventing such phenomena but they are not very effective.